Field of the Invention
The present invention relates to a thermoelectric element using semiconductive ceramic materials, and more particularly, it relates to an improvement in a mode of electrically connecting a plurality of semiconductor members which are made of semiconductive ceramic materials with each other.
Description of the Prior Art
It is widely known that various substances have thermoelectric power based on Seebeck effects. An electromotive force generated by such thermoelectric power may be utilized in a thermocouple, a thermoelectric element or the like, and these have already been widely put to practical use. Examples of materials having characteristic values which are suitable for such practical use are an intermetallic compound, a semimetal compound and a semiconductor.
Among these, semiconductor are higher in thermoelectric power than the other materials, and are suitable for use in a thermoelectric element in a power supply. Such a semiconductor has either a positive or negative electromotive force, depending on the polarity of the majority carriers contained therein. For example, a hot side is positive in the case of n-type carriers and negative in the case of p-type carriers. It is well known to join an n-type semiconductor material with a p-type semiconductor material to provide a temperature contact at the junction therebetween, thereby to increase the thermoelectromotive force which is delivered as a whole.
For example, oxides of iron and silicon are mixed to form an iron-silicon compound to semiconductorize the same and form a p-n junction, by mixing pulverulent raw materials with each other, pressure molding, and then sintering, thereby obtaining such a p-n junction. On the other hand, a semiconductor manufacturing process may be used to form a p-n junction on a silicon substrate, through a technique such as ion implantation or CVD.
In the case of semiconductive ceramic, materials however, it is generally difficult to form a junction between a p-type semiconductive ceramic material and an n-type semiconductive ceramic material, unlike in the aforementioned examples of the semiconductor materials other than semiconductive ceramic materials.
One problem that may occur is that, in the case of a semiconductive ceramic material which is obtained by semiconductorizing metallic atoms of metal oxide ceramic in an excess or deficit state, a layer of the original metal oxide may be defined at the p-n junction portion, and this will greatly increase the resistance of the element, such that the element cannot be applied to practical use. Also in a semiconductive ceramic materials which is semiconductorized by valency control, a p-n junction may not be sufficiently formed due to diffusion of additives.
The foregoing description has concerned the case of forming a p-n junction by a step of firing semiconductive ceramic materials. On the other hand, however, semiconductive ceramic materials may also be electrically connected with each other in a later step, after they have already been fired, to implement a p-n junction through such connection. However, fired semiconductive ceramic materials cannot be directly subjected to soldering, and hence any such soldering step must be carried out after performing metallization by appropriate means.
For example, FIG. 11 shows a device disclosed in Japanese Patent Laying-Open Gazette No. 114090/1979, in which a plurality of semiconductive ceramic members 1 to 4 of reduction type titanium oxide are respectively provided with ohmic electrodes 5 to 12, by a method such as vapor deposition or thick film printing. The electrodes 5 to 12 are sequentially soldered through lead wires 13 to 17 to connect the semiconductive ceramic members 1 to 4 in series with each other, in order to increase the total output by this multistage series connection of the semiconductive ceramic members 1 to 4. In this prior art example, therefore, the operation for electrically connecting the semiconductive ceramic members 1 to 4 is complicated, and the element obtained cannot be compact.
Generally speaking, there are two main uses for such thermoelectric elements. Thermoelectric elements are of great importance for use in a power supply, and hence various efforts have been made to increase the thermoelectromotive force which is delivered by the element as a whole.
On the other hand, a thermoelectromotive force generated by a Seebeck effect can also be put to use in a temperature detecting element, for example. A thermocouple is typically used as such a temperature detecting element. The thermocouple is adapted to utilize the fact that a thermoelectromotive force generated across a junction between two types of different metals varies with the junction temperature. Such thermoelectromotive force, being 40 to 80 .mu.V/K, for example, is relatively small since the Seebeck factors of metals are generally small. Nevertheless, such a thermocouple has been widely used since the temperature coefficient of the thermoelectromotive force is relatively small and hence the temperature coefficient can be easily corrected.
In recent years, microprocessor-based techniques have been applied in the field of temperature and temperature difference measurement, whereby a measuring apparatus of high performance can be provided at a low cost. In particular, the performance of the signal processing systems in such measuring system has been greatly improved. Hence, improvement in the sensitivity of the detecting element would also be highly desirable. However, the thermoelectromotive force generated by such a conventional thermocouple has heretofore been insufficient, in view of this improvement in sensitivity.
On the other hand, thermoelectric elements have been developed mainly for use in supplies power, as hereinabove described. A material for such an element is selected from the system of Bi.sub.2 Te.sub.3, Bi.sub.2 Sb.sub.2 Te.sub.15, FeSi, Si-Ge and the like, for example. These materials, having relatively large Seebeck factors of 0.2 to 0.6 mV/K, are apparently suitable for providing highly sensitive temperature detecting elements.
However, such a thermoelectric element developed for use in a power supply is generally to be used in a high temperature range of 300.degree. to 800.degree. C., and furthermore, such thermoelectric elements are to be provided with large temperature gradients, i.e., a temperature difference of hundreds of degrees. Further, efforts toward increasing the Seebeck factor have been merely directed toward improving the conversion efficiency. In the type of thermoelectric element developed for use in a power supply, therefore, no consideration is generally given to linearity in the temperature coefficient. Further, the mechanical strength thereof is rather insufficient. Accordingly, the present inventors have realized that there is a need for further improvement in the sensitivity of a thermoelectric element for use in temperature detection.
It is well know in the art that semiconductive ceramic material is larger in thermoelectromotive force than metal by at least an order of magnitude. For example, it has been reported by Saburi, in the Journal of the Physical Society of Japan, 1959, that BaTiO.sub.3 has a thermoelectromotive force of 860 .mu.V/K. Such a semiconductive ceramic may contribute to further improvement in the sensitivity of a temperature detecting element since it has passable mechanical strength and is capable of generating a thermoelectromotive force which is higher than that of the aforementioned materials that were developed for use in a power supply. However, semiconductive ceramic also has the problem that the temperature coefficient of thermoelectromotive force is relatively high. Thus, it is difficult to directly apply the semiconductive ceramic for use in a highly sensitive temperature detecting element, but some temperature coefficient correcting means is required.